Hello,

Sorry about the radio silence from me --
I've been trying to get this working on my end as well.

I was able to get my passed-through USB device  on a qemu system to
have a firmware_node by
using the "Upgrading ACPI tables via initrd" kernel feature [1]. In
case this provides helpful information,
the below describes what I did.

This was using the default yocto core-image-minimal image and
qemu-system-x86_64.

I invoke qemu with the convenience "runqemu" script, as follows:
runqemu nographic qemuparams="
    -initrd ../acpi-overlay/instrumented_initrd
    -device 'usb-host,vendorid=0x10c4,productid=0xea90'
    -pflash ./build/tmp/work/core2-64-poky-linux/ovmf/edk2-stable202211-r0/ovmf/ovmf.fd
    "

Which invokes qemu with something like the following (sorry about the
long lines..):
qemu-system-x86_64 \
    -device virtio-net-pci,netdev=net0,mac=52:54:00:12:34:02 \
    -netdev tap,id=net0,ifname=tap0,script=no,downscript=no \
    -object rng-random,filename=/dev/urandom,id=rng0 \
    -device virtio-rng-pci,rng=rng0 \
    -drive file=/home/kaehnd/src/local/x86/build/tmp/deploy/images/qemux86-64/core-image-minimal-qemux86-64-20230306143252.rootfs.ext4,if=virtio,format=raw
\
    -usb -device usb-tablet -usb -device usb-kbd   -cpu IvyBridge \
    -machine q35,i8042=off -smp 4 -m 256 \
    -device 'usb-host,vendorid=0x10c4,productid=0xea90' \
    -serial mon:stdio -serial null -nographic \
    -kernel /home/kaehnd/src/local/x86/build/tmp/deploy/images/qemux86-64/bzImage
\
    -append 'root=/dev/vda
        rw  ip=192.168.7.2::192.168.7.1:255.255.255.0::eth0:off:8.8.8.8
        console=ttyS0 console=ttyS1 oprofile.timer=1
        tsc=reliable no_timer_check rcupdate.rcu_expedited=1 '


The sysfs path tree for the CP2112 was as follows:
#> ls -l  /sys/bus/hid/devices/0003:10C4:EA90.0003
lrwxrwxrwx    1 root     root             0 Mar  6 19:24
/sys/bus/hid/devices/0003:10C4:EA90.0003 ->
../../../devices/pci0000:00/0000:00:1d.1/usb3/3-1/3-1:1.0/0003:10C4:EA90.0003


Out of the box, firmware_node files existed only through what I assume
is the PCI bus:
/sys/devices/pci0000:00

It's ACPI path:
#> cat /sys/devices/pci0000:00/firmware_node/path
\_SB_.PCI0

Using the instructions at [1], I grabbed the dsdt table, and modified
it as follows.

Underneath the PCI0 node, I added the following ASL

Device (SE9)
{
    Name (_ADR, 0x001D0001) // _ADR: Address
    Device (RHUB)
    {
        Name (_ADR, Zero)
        Device (CP2) // the USB-hid & CP2112 shared node
        {
            Name (_ADR, One)
        }
    }
}

If I'm understanding correctly, this adds the SE9 device as function 1
of PCI device 0x1d,
then RHUB as the USB controller it provides, and finally, CP2 as the
USB device attached to port 1 of the controller.

With this as the loaded dsdt table, the USB device now has a firmware_node :)
#> cat /sys/bus/usb/devices/3-1:1.0/firmware_node/path
\_SB_.PCI0.SE9_.RHUB.CP2_

After applying my patches, the HID device also references this node:
#> cat /sys/bus/hid/devices/0003:10C4:EA90.0003/firmware_node/path
\_SB_.PCI0.SE9_.RHUB.CP2_

With this all said -- I noticed iasl prints this statement when trying
to create a node with a lowercase name:
"At least one lower case letter found in NameSeg, ASL is case
insensitive - converting to upper case (GPIO)"

I wonder if this suggests that adding a call to toupper() to
acpi_fwnode_get_named_child_node would be
an appropriate solution for the node name casing issue....

[1] https://www.kernel.org/doc/html/latest/admin-guide/acpi/initrd_table_override.html